1. Field of the Invention
The present invention relates to a loop communication system, a communication device, and a loop communication method in a network constituted by a serial bus such as IEEE 1394 which is employed in digital products such as a personal computer and a digital home electrical appliance.
2. Description of the Related Art
A communication system based on IEEE 1394b is conventionally known. FIG. 1 is a block diagram showing a configuration of the conventional communication system based on IEEE 1394b. Referring to FIG. 1, the communication system includes three communication units 100-i (i=1, 2, 3). Each unit 100-i is provided a port 1 (Beta Port1) 10-i, a port 2 (Beta Port2) 20-i, a connector 30-i, a connector 40-i, a transmission/reception system (TX/RX System) circuit 50-i, a link layer control circuit 60-i, an application/transaction layer control circuit 70-i. The connector 30-i is provided for the port 1 (Beta Port1) 10-i, and similarly, the connector 40-i is provided for the port 2 (Beta Port2) 20-i. The transmission/reception system (TX/RX System) circuit 50-i is located at a logically upper position of the port 1 (Beta Port1) 10-i and the port 2 (Beta Port2) 20-i. Also, the link layer control circuit 60-i is located at a logically upper position of the transmission/reception system circuit 50-i, and the application/transaction layer control circuit 70-i is located at a logically upper position of the link layer control circuit 60-i. The transmission/reception system (TX/RX System) circuit 50-i includes an arbitration state machine (STM) 51-i to manage a transmission right on a bus. Also, the communication units 100-1 and 100-2 are connected between connectors 40-1 and 30-2 through a bi-directional cable to allow dual transmission/reception, and the communication units 100-2 and 100-3 are connected between connectors 40-2 and 30-3 through a bi-directional cable to allow dual transmission/reception. Thus, a network of communication units 100-1, 100-2 and 100-3 is
The Beta Port1 10-i is provided with a beta port state machine 11-i for executing management of synchronization with a connection destination, a scrambler 12-i, a de-scrambler 13-i, an 8B10B coding circuit 14-i, a 10B8B coding circuit 15-i, a driver 16-i and a receiver 17-i. The Beta Port2 20-i is provided with a beta port state machine 21-i for executing management of synchronization with a connection destination, a scrambler 22-i, a de-scrambler 23-i, an 8B10B coding circuit 24-i, a 10B8B coding circuit 25-i, a driver 26-i and a receiver 27-i. 
As described above, the port 2 (Beta Port2) 20-1 of the unit 1 100-1 is connected through the cable to the port 1 (Beta Port1) 10-2 of the communication unit 2 100-2, and the port 2 (Beta Port2) 20-2 of the communication unit 2 100-2 is similarly connected through the cable to the port 1 (Beta Port1) 10-1 of the communication unit 3 100-3. In the dual communication system defined based on IEEE 1394b, both of the transmission and the reception are carried out through the single cable. Thus, a transmission line and a reception line exist in one cable. Also, when the network is built up from the three or more units, the unit having the two or more ports is necessarily required as the communication unit 2 100-2 in FIG. 1. In this way, the requirements of the two ports and the cable having the two lines are constraint when the space saving and the cost down are aimed.
The mechanism of the data transmission/reception will be described below. When data is transmitted from the application control circuit 70-i to a different unit through the cable, the arbitration state machine 50-i arbitrates a transmission right. Then, after obtaining the transmission right, the communication unit 100-i transmits the data onto the cable through the port (Beta Port) 20-i and the connector 40-i. At the time of the reception, data is received through the cable and the connector 40-i to the port (Beta Port) 20-i, and is sent to the application control circuit 70-i. As described above, in the communication network in which the three communication units 100-1, 100-2 and 100-3 are connected, the data transmitted from the port 2 (Beta Port2) 20-1 of the communication unit 1 100-1 is received through the transmission/reception dual cable by the port 1 (Beta Port1) 10-2 of the communication unit 2 100-2. At the same time, in the communication unit 2 100-2, the data is transferred to the port 2 (Beta Port2) 20-2, and is transmitted through the transmission/reception dual cable to the communication unit 3 100-3. The communication unit 3 100-3 carries out the reception of the data by the port 1 (Beta Port1) 10-3.
Next, the operation at the time of communication will be described below with reference to FIGS. 2 to 5. This operation is in case of the IEEE 1394b (a conventional technique with no loop structure). FIG. 2 shows transmission and reception operations. An output request data is outputted from the application control circuit 70-i of the unit 100-i to the port 1 (Beta Port1) 10-i and the port 2 (Beta Port2) 20-i. Also, at the same time, an arbitration code or arbitration signal is received from the port 1 (Beta Port1) 10-i and the port 2 (Beta Port2) 20-i to obtain the transmission right. FIG. 3 shows data formats in this case. When the data is transmitted to the cable side, a data prefix is transmitted prior to the data, and a data end is transmitted after the data. FIG. 4 shows reception and transfer functions. At the time of the data reception, at the same time as the transmission of the data to the application side or the link side, the data is transferred to the port 2 (Beta Port2) 20-i. When the arbitration code signal is received from the port 2 (Beta Port2) 20-i, the arbitration of the transmission right is carried out by the transmission/reception system (TX/RX System) circuit 50-i, and the arbitration code signal is transmitted to the port 1 (Beta Port1) 10-i. FIG. 5 shows data formats in FIG. 4. The data prefix and the data end has been added to the transmission/reception data.
The mechanism of the transmission right arbitration will be described below. FIG. 6 is a diagram showing a flow of request in a conventional configuration. FIG. 7 is a diagram showing a flow of grant in the conventional configuration. The system has a unit A 100-11, a unit B 100-12, a unit C 100-13, and a unit D 100-14 as an example. It is assumed that a request is issued from the unit B 100-12 and the unit D 100-14 as the arbitration code. Here, the unit A 100-11 is a root that can determine distribution of the transmission right. If the transmission right is given to the unit D 100-14, the data prefix is indicated to the unit B 100-12, and a grant signal is sent to the unit D 100-14. Consequently, the unit D 100-14 obtains the transmission right.
In FIG. 1, the port 1 (Beta Port1) 10 of the communication unit 1 100-1 and the port 2 (Beta Port2) 20 of the communication unit 3 100-3 are not used. Also, the total number of the transmission/reception lines is (Cable Transmission/Reception Pair (Two Lines)×2=4 lines. In order to miniaturize the circuit scale and down the cost, the excessive ports and transmission/reception lines are required to be omitted.
Also, in the IEEE 1394b, a network is defined in which synchronization is established in the port (Beta Port). Thus, although a loop connection is possible, the data communication is impossible because of automatic separation. That is, even if the loop is formed physically, the loop is not formed on the communication because the loop is separated.
Moreover, in the IEEE 1394b, under the management by the state machine shown in FIGS. 8A and 8B, the port (Beta Port) has a synchronization establishing function. When the loop is configured in the synchronous communication such as the IEEE 1394b, the same data continues to be repeatedly transmitted, and a folded communication is impossible for the arbitration code (arbitration signal). Thus, a trouble occurs. For example, the Grant for the Request cannot be accurately issued.
In conjunction with the above description, a transmission system is disclosed in Japanese Laid Open Patent Publication (JP-A-Heisei, 11-32067), in which a fault frame can be surely removed when a network of a loop structure is used. In this conventional transmission system, a plurality of nodes are coupled to a transmission path, and data is mutually transmitted and received through the transmission path. The node carries out a transmission process on a communication frame having a transmission data to assign a node passage identifier indicative of the number of frames passing through the node, in addition to a reception node identifier. On the other hand, the node carries out a reception process when the received communication frame is destined for the node itself. Also, in the node, a transmission/reception processing unit carries out a transmission process for relaying the communication frame on the transmission path if the received communication frame is not destined for the node itself. Also, a trouble frame discarding unit updates a value of the node passage identifier of the received communication frame, and stops the relaying if the number of frames is determined to reach a preset number from the value. In this way, this conventional example controls transmission by counting the reception data and is applicable to the configuration of FIG. 1.